Michel Hamelin

Structure of Titan's low altitude ionized layer from the Relaxation Probe onboard HUYGENS

Some of the secrets of the atmosphere of Titan have been unveiled by the Huygens Probe. The Permitivity Wave and Altimetry system detected a hidden ionosphere much below the main ionosphere, that lies between 600 and 2000 km. Theoretical models predicted a low altitude ionosphere produced by cosmic rays that, contrary to magnetospheric particles and UV photons, are able to penetrate down in the atmosphere. Two sensors: Mutual Impedance (MI) and Relaxation Probe (RP) measured the conductivity of the ionosphere by two different methods and were able to discriminate the two branches of electrical conductivity due to the positive and negative charges. The measurements were made from 140 to 40 km and show a maximum of charge densities ≈2 × 109 m−3 positive ions and ≈450 × 106 m−3 electrons at around 65 km. Here we present the altitude distribution of the concentration of positive ions and electrons obtained from the RP and MI sensors.

Electrical properties and porosity of the first meter of the nucleus of 67P/Churyumov-Gerasimenko - As constrained by the Permittivity Probe SESAME-PP/Philae/Rosetta

Comets are primitive objects, remnants of the volatile-rich planetesimals from which the solar system condensed. Knowing their structure and composition is thus crucial for the understanding of our origins. After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko in November 2014, for the first time, the Rosetta mission provided the opportunity to measure the low frequency electrical properties of a cometary mantle with the permittivity probe SESAME-PP (Surface Electric Sounding and Acoustic Monitoring Experiment−Permittivity Probe). Aims. In this paper, we conduct an in-depth analysis of the data from active measurements collected by SESAME-PP at Abydos, which is the final landing site of Philae, to constrain the porosity and, to a lesser extent, the composition of the surface material down to a depth of about 1 m. Methods. SESAME-PP observations on the surface are then analyzed by comparison with data acquired during the descent toward the nucleus and with numerical simulations that explore different possible attitudes and environments of Philae at Abydos using a method called the Capacity-Influence Matrix Method. Results. Reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the Rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at Abydos to be >2.45 ± 0.20, which is consistent with a porosity <50% if the dust phase is analogous to carbonaceous chondrites and <75% in the case of less primitive ordinary chondrites. This indicates that the near surface of the nucleus of 67P/Churyumov-Gerasimenko is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer.

Comment on "An analysis of VLF electric field spectra measured in Titan's atmosphere by the Huygens probe" by J. A. Morente et al.

Citation: Grard, R., S. Berthelin, C. Beghin, M. Hamelin, J.‐J. Berthelier, J. J. Lopez‐Moreno, and F. Simoes (2011), Commenton “An analysis of VLF electric field spectra measured in Titan’s atmosphere by the Huygens probe” by J. A. Morente et al.,J. Geophys. Res., 116, E05005, doi:10.1029/2009JE003555.

Electron temperature anisotropy associated to field-aligned currents in the Earth's magnetosphere inferred from Rosetta MIP-RPC observations during 2009 flyby

A new approach is proposed for data interpretation of the Mutual Impedance Probe (MIP) instrument from the Rosetta Plasma Consortium (RPC) during the 2009 Earths fly-by gravity assist through the magnetosphere, from dusk to dawn regions. The spacecraft trajectory of +/-8 Re (Earths radius) was crossing several structures of field aligned currents (FACs) and radiations belts on both legs of the closest approach (CA, 2.450 km altitude). As routinely revealed by several pioneering space missions, natural and forced electrostatic wave emissions called Fqs were observed over +/- 3 Re at around CA using a dedicated mode of the MIP instrument. These emissions are lying between consecutive harmonics of the electron-cyclotron frequency, and their wavelength is perpendicular to the magnetic field-lines. Provided that the Fqs wavelengths projected along the MIP antenna might be estimated, it is shown that the local value of the Larmor radius can be deduced, hence the electron temperature component perpendicular to the magnetic field is subsequently derived. On the other hand, during the time of Fqs observations, the presence of VLF hiss emissions usually observed in these regions, gives us the possibility to determine the electron temperature anisotropy associated to the electrostatic electron anisotropy instability according to the theoretical model proposed by Gary and Cairns (JGR, vol.104,1999). Significant dynamic constraints revealed by crossing successive series of FACs tubes are shown being controlled by this anisotropy, and the fact that the magnetic pressure is significantly larger than the thermal pressure suggests that the FACs lobes are non-force free.